High-pressure treatment (HPT) has been shown to induce significant levels of flocculation in model
oil-in-water emulsions stabilized by β-lactoglobulin at neutral pH, as indicated by changes in droplet
size distribution and rheological behavior. Light microscopy has also provided additional evidence
of extensive droplet flocculation following severe treatment. The proportion of unadsorbed protein
greatly influences the extent of flocculation, and this may explain in part the nature of these pressure-induced effects. It has been observed that severe HPT (800 MPa for 60 min) is equivalent to relatively
mild thermal treatment (TT) (65 °C for 5 min) in terms of the associated changes in emulsion gel
rheology. Since HPT destabilizes these emulsion systems to a much lesser degree than TT, it can
be considered to be a gentler processing operation in comparison. Emulsion flocculation is more
sensitive to pressure and temperature at pH values closer to the isoelectric point and at higher
ionic strength. That is, conditions favoring a loss of electrostatic stability tend to cause an increase
in sensitivity toward pressure and temperature, although HPT consistently appears to be a gentler
process than TT under all conditions studied.
Keywords: High-pressure treatment; thermal treatment; protein functionality; flocculation; rheology;
emulsion stability; emulsion gel
The effect of high-pressure treatment (HPT) on the droplet-size distribution and small-deformation rheology of oil-in-water emulsions containing beta-lactoglobulin and a nonionic surfactant or sodium caseinate has been investigated at neutral pH. Addition of Tween 20 (polyoxyethylenesorbitan monolaurate) to a beta-lactoglobulin-stabilized emulsion results in competitive displacement of the adsorbed globular protein film and, following HPT, the formation of a less flocculated emulsion. The age of the beta-lactoglobulin-stabilized emulsion prior to addition of sodium caseinate influences the competitive adsorption behavior. The strengthening of the beta-lactoglobulin layer with time makes it more resistant to disruption by sodium caseinate. The level of pressure-induced flocculation of beta-lactoglobulin-coated oil droplets depends on the intensity of processing conditions and on the degree of interfacial displacement. In contrast, beta-lactoglobulin added after emulsification appears to show little evidence of competitive adsorption behavior at the caseinate oil-water interface. Changes in the rheological properties of these latter systems following HPT can be attributed to pressure-induced denaturation and gelation of beta-lactoglobulin in the continuous phase of the emulsion.
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